The present invention is directed to improvements in obtaining accurate measurements from sensors of the type utilizing relatively movable electromagnetic coils and ferromagnetic cores. More particulary, the present invention is directed to an improvement in such a sensor when used in measuring load forces applied to a structural member.
While the invention may find utility in a broad variety of applications, the disclosure will be facilitated by reference to the specific problem of monitoring the compressive force applied to a quantity of forage material or hay in a baling apparatus. The problem of accurately monitoring and controlling the compressive force applied during the baling of forage material or hay by such a baling machine has been heretofore accomplished only with relative difficulty and expense.
In such a baling machine, it is generally desirable to achieve relatively uniform density in the bales of forage material or hay produced. The bale density is in turn controlled by the amount of axial force applied to the bale by an axial compression member driven by one or more elongate tubular "plunger arms". The compression on this plunger arm is further grounded in accordance with the position of movable side wall members of the baling apparatus. These side wall members, together with fixed top and bottom walls, define what is generally referred to in the art as the baling chamber
In accordance with prior art practice, various means were utilized to measure the compressive force applied in the axial direction by the plunger arms and to control the positions of the movable sidewalls accordingly so as to achieve relatively uniform bale density. See, for example, U.S. Pat. Nos. 4,148,254; 4,166,414 and 4,168,659 assigned to the assignee of the present invention. This practice allowed the system to "open up" the sidewalls, if necessary to prevent excessive load on the apparatus and consequent damage thereto. However, as previously noted, solutions heretofore proposed for the problem of measuring the axial compressive force or load have been relatively difficult and expensive to implement.
For example, presently available stress guages and related circuitry for obtaining accurate measurements of this compressive force or load are relatively complex and expensive. This is due in large part to the non-linear characteristics of the stresses or compressive forces experienced in the plunger arm member in response to various compressive forces applied thereto. In this regard, the plunger arm generally comprises a tubular rectilinear structural member which is utilized to drive a movable compression surface longitudinally of the compression chamber. In practice, the plunger may apply a compressive force of up to 100,000 pounds to the forage material. However, during this compression the plunger arm itself is in fact compressed only a very small amount, for example on the order of 0.01 inches.
It is generally known to measure relative positions or movements of this type by the use of a coil of wire into which a ferromagentic core extends. In the case of a plunger arm, both the coil and ferromagnetic core may be mounted to opposing end surfaces of the elongate tubular plunger arm. Accordingly, a change in flux density of the coil will take place as the ferromagentic core is moved relative thereto in response to the compression of the plunger arm.
The coil of wire is often coupled in an electronic circuit to form an oscillator, the frequency of which varies with the movement of the core inside of the coil of wire. However, it will be appreciated that with a compression of only on the order of 0.01 inches only relatively small changes in and flux density occur. Hence, a high degree of resolution in reading the corresponding changes in the electrical signals developed across the coil is required.
Additionally, due in part to the rectilinear structure of the tubular plunger arm, the relationship between the force applied thereto and the amount of axial compression is a non-linear one. Moreover, the relationship between movement of the core piece inside the coil of wire and frequency developed by the oscillator is also non-linear. Accordingly, each such sensor must be carefully calibrated with the particular plunger on which it is to be used. It will be appreciated that such individual calibration can become relatively cumbersome and expensive.
In accordance with the present invention, however, a novel and relatively inexpensive method and apparatus for achieving this individual calibration of sensors has been discovered. Moreover, as will be seen from the ensuing description of the invention, a number of other advantages and novel features are achieved by the apparatus and method of the invention.